The present invention generally relates to the field of demultiplexers and, more particularly, to a demultiplexer that includes a grating assembly formed on a first side thereof and a photodetector assembly formed on a second side thereof which is opposite the first side.
Various types of demultiplexer designs have been proposed. One common configuration is to have the grating that provides the separation function within an interior location of the demultiplexer, such as within a waveguide that is the optical carrier for the multiplexed or combined optical signal. Gratings of this type require at least some type of crystal regrowth. Crystal regrowth is a complex and difficult process, and the result of any such regrowth may yield a demultiplexer that suffers one or more types of deficiencies in its performance.
Another common configuration for a demultiplexer provides a demultiplexing function which divides all N wavelengths among N separate detection channels (i.e., the multiplexed signal is split and sent to N separate detection channels). The Mth signal channel thereby rejects the other Nxe2x88x921 wavelengths in its channel and detects only the Mth wavelength. Thus (Nxe2x88x921)/N portion of the original signal is rejected. This has an adverse impact on the power demands for the demultiplexer.
A first aspect of the present invention is embodied by a particular demultiplexer design. This demultiplexer includes first and second waveguides that may be viewed as being in a stacked configuration, with the second waveguide being disposed a higher elevation in the stack than the first waveguide. xe2x80x9cStackedxe2x80x9d does not necessarily mean that the second waveguide is directly above the first waveguide or vice versa, although this typically will be the case. A grating assembly is disposed on either the upper or the lower surface of the stack, but nonetheless on the first waveguide.
Various refinements exist of the features noted in relation to the first aspect of the present invention. Further features may also be incorporated in the first aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. Both the first and second waveguides are optical conduits of sorts. Multiplexed or combined optical signals may be directed into the first waveguide for demultiplexing by the grating assembly, such that the first waveguide may be characterized as an input channel. These individual demultiplexed optical signals (e.g., of individual wavelengths) may be directed into the second waveguide for a readout of the same, such that the second waveguide may be characterized as a plurality of output channels. Each of these output channels may be associated with a single individual wavelength or a relatively narrow band of wavelengths that is on the order of 1-2 nanometers wide. Hereafter, any discussion of xe2x80x9cindividual wavelengthsxe2x80x9d progressing to the second waveguide for readout encompasses both individual wavelengths and this relatively narrow wavelength band on the order of 1-2 nanometers wide.
Definition of these plurality of output channels in the second waveguide may be accomplished by having the second waveguide actually be in the form of a plurality of second waveguide sections that are spaced in a direction in which the combined optical signal progresses through the first waveguide. A photodetector may be attached to/formed on/in each of these second waveguide sections for actually accomplishing this readout. A photodetector may be part of its corresponding second waveguide section (i.e., a given photodetector may be smaller than its corresponding second waveguide section), or may in fact define the entirety of its corresponding second waveguide section. It may be possible to utilize a continuous second waveguide with a plurality of photodetectors that are spaced in the direction in which the combined optical signal is progressing through the first waveguide for other applications of the structure associated with the first aspect, although there may be difficulties with this configuration for the subject demultiplexing application. Preferably, a photodetector assembly is integrally formed in association with the second waveguide, including defining the entirety of the second waveguide (e.g., a given photodetector may define the entirety of its corresponding second waveguide section) and defining only a portion of the second waveguide in which case the photodetector would be disposed on/extend within the second waveguide. As such, a photodetector assembly is effectively disposed on one side of the stack, and the grating assembly is disposed on the opposite side of the stack to provide a dual-side demultiplexer.
What may be characterized as a barrier layer or index control channel may be and preferably is disposed between the first and second waveguides in the case of the first aspect. In the case where the second waveguide is continuous, the barrier layer would be continuous. Where the second waveguide is in the form of a plurality of spaced second waveguide sections, the barrier layer would typically be in the form of a plurality of spaced barrier layer sections (e.g., such that a barrier layer section is disposed between each second waveguide section and the first waveguide), although it could still be a continuous structure as well. Selective coupling of the first and second waveguides effectively is the function of the barrier layer. Functionally, the barrier layer allows only certain light to pass from the first waveguide into the second waveguide in a particular region of the second waveguide. The remainder of the light is thereby prohibited from progressing from the first waveguide into this region of the second waveguide. Consider the case where the second waveguide is in the form of a plurality of spaced second waveguide sections, and where the barrier layer is similarly in the form of a plurality of spaced barrier layer sections. Light of a certain wavelength may be directed from the grating assembly associated with the first waveguide toward a particular barrier layer section and its overlying second waveguide section. This particular barrier layer section allows this certain wavelength light to progress through the barrier layer section and into its overlying second waveguide section. All other light is prohibited from passing through this barrier layer section into its overlying second waveguide section. One may view the grating assembly as not only separating out a particular wavelength from a combined or multiplexed optical signal, but xe2x80x9cprocessingxe2x80x9d this particular wavelength of light into a form which will allow the same to pass from the first waveguide, through the barrier layer, and into its overlying second waveguide section.
The first and second waveguides may be characterized as being asynchronous in the first aspect. Furthermore, the first and second waveguides may and typically will have different refractive indices, may and typically will have different thicknesses, or both. As noted above, the second waveguide may be a continuous structure or may be in the form of a plurality of second waveguide sections that are spaced in a direction in which a multiplexed or combined optical signal is progressing through the first waveguide.
The function of the grating assembly is to somehow separate out the individual optical signals from a combined or multiplexed signal in the case of the first aspect. Preferably this is done by principles of reflection versus transmission. That is, preferably the grating assembly of the first aspect is a reflective grating versus a transmissive grating. The grating assembly may include a plurality of grating subassemblies that are spaced in a direction in which a multiplexed or combined optical signal is progressing through the first waveguide. Each grating subassembly may be wavelength specific (or specific to a given bandwidth as noted). Furthermore, each grating subassembly may account for both polarities of light. One portion of each grating subassembly may be configured to direct light of its associated wavelength and of a first polarity toward the second waveguide. Another portion of each grating subassembly may be configured to direct light of its associated wavelength and of a second, different polarity toward the second waveguide. These portions will typically be aligned in a direction in which a multiplexed or combined optical signal is progressing through the first waveguide. As such, a particular grating subassembly will be specific to a given wavelength, and will account for both polarities of this given wavelength.
One way in which both polarities of light may be accounted for by each grating subassembly is by using a slightly different grating spacing within each grating subassembly. Consider the case where there are a plurality of grating subassemblies, and where each grating subassembly includes first and second grating sections that account for the first and second polarities of light, respectively. The grating spacing used in the first grating section of each grating subassembly may be within a range of about 0.1 to about 0.5 microns different from the grating spacing used in the second grating section of the same grating subassembly. Stated another way, the grating spacing used in the first and second grating sections of each grating subassembly differ by an amount that is of a range of 0.1 microns to about 0.5 microns. Adjacent grating subassemblies will have larger grating spacings to account for addressing different wavelengths of light. The grating spacing used by adjacent grating subassemblies will typically be at least about 1 micron different from each other.
A second aspect of the present invention is embodied by a particular demultiplexer design. This demultiplexer includes first and second waveguides. A grating assembly is associated with the first waveguide. The function of this grating assembly is to somehow separate out the individual optical signals from a combined or multiplexed signal. Both polarities of light are accounted for by the grating assembly associated with the second aspect.
Various refinements exist of the features noted in relation to the second aspect of the present invention. Further features may also be incorporated in the second aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. Initially, the second aspect of the present invention may be used in combination with the above-noted first aspect.
Multiplexed or combined optical signals may be directed into the first waveguide for demultiplexing by the grating assembly of the second aspect, such that the first waveguide may be characterized as an input channel. These individual demultiplexed optical signals (e.g., of individual wavelengths) may be directed into the second waveguide for a readout of the same, such that the second waveguide may be characterized as a plurality of output channels. Each of these output channels may be associated with a single individual wavelength. Definition of these plurality of output channels in the second waveguide may be accomplished by having the second waveguide actually be in the form of a plurality of second waveguide sections that are spaced in a direction in which the combined or multiplexed optical signal progresses through the first waveguide. A photodetector may be attached to/formed on/in each of these second waveguide sections for actually accomplishing this readout. A photodetector may be part of its corresponding second waveguide section (i.e., a given photodetector may be smaller than its corresponding second waveguide section), or may in fact define the entirety of its corresponding second waveguide section as noted. It may be possible to utilize a continuous second waveguide with a plurality of photodetectors that are spaced in the direction in which the combined optical signal is progressing through the first waveguide for other applications of the structure associated with the second aspect, although there may be difficulties with this configuration for the subject demultiplexing application. Preferably, a photodetector assembly is integrally formed in association with the second waveguide (including defining the entirety of the second waveguide (e.g., and a given photodetector may define the entirety of its corresponding second waveguide section) and defining only a portion of the second waveguide in which case the photodetector would be disposed on/extend within the second waveguide), and the grating assembly is integrally formed on a side of the first waveguide that is opposite that which projects toward the second waveguide. As such, a photodetector assembly is effectively disposed on one side of a demultiplexing stack, and the grating assembly is disposed on the opposite side of this demultiplexing stack to provide a dual-sided demultiplexer.
What may be characterized as a barrier layer or index control channel may be and preferably is disposed between the first and second waveguides. In the case where the second waveguide is continuous, the barrier layer would be continuous. Where the second waveguide is in the form of a plurality of spaced second waveguide sections, the barrier layer would typically be in the form of a plurality of spaced barrier layer sections (e.g., such that a barrier layer section is disposed between each second waveguide section and the first waveguide), although it could still be a continuous structure as well. Selective coupling of the first and second waveguides effectively is the function of the barrier layer. Functionally, the barrier layer allows only certain light to pass from the first waveguide into the second waveguide in a particular region of the second waveguide. The remainder of the light is thereby prohibited from progressing from the first waveguide into this region of the second waveguide. Consider the case where the second waveguide in the form of a plurality of spaced second waveguide sections, and where the barrier layer is similarly in the form of a plurality of spaced barrier layer sections. Light of a certain wavelength may be directed from the grating assembly associated with the first waveguide toward a particular barrier layer section and its overlying second waveguide section. This particular barrier layer section allows this certain wavelength light to progress through the barrier layer section and into its overlying second waveguide section. All other light is prohibited from passing through this barrier layer section into its overlying second waveguide section. One may view the grating assembly as not only separating out a particular wavelength from a combined or multiplexed optical signal, but xe2x80x9cprocessingxe2x80x9d this particular wavelength of light into a form which will allow the same to pass from the first waveguide, through the barrier layer, and into its overlying second waveguide section.
The first and second waveguides may be characterized as being asynchronous in the second aspect. Furthermore, the first and second waveguides may and typically will have different refractive indices, may and typically will have different thicknesses, or both. As noted above, the second waveguide may be a continuous structure or may be in the form of a plurality of second waveguide sections that are spaced in a direction in which a multiplexed or combined optical signal is progressing through the first waveguide.
The function of the grating assembly is to somehow separate out the individual optical signals from a combined or multiplexed signal in the case of the second aspect. Preferably this is done by principles of reflection versus transmission. That is, preferably the grating assembly of the second aspect is a reflective grating versus a transmissive grating. The grating assembly of the second aspect may include a plurality of grating subassemblies that are spaced in a direction in which a multiplexed or combined optical signal is progressing through the first waveguide. Each grating subassembly may be wavelength specific (or specific to a given bandwidth as noted). Furthermore, each grating subassembly accounts for both polarities of light as noted. One portion of each grating subassembly may be configured to direct light of its associated wavelength and of a first polarity toward the second waveguide. Another portion of each grating subassembly may be configured to direct light of its associated wavelength and of a second, different polarity toward the second waveguide. These portions will typically be aligned in a direction in which a multiplexed or combined optical signal is progressing through the first waveguide. As such, a particular grating subassembly will be specific to a given wavelength, and will account for both polarities of this particular wavelength.
One way in which both polarities of light may be accounted for by each grating subassembly is by using a slightly different grating spacing within each grating subassembly. Consider the case where there are a plurality of grating subassemblies, and where each grating subassembly includes first and second grating sections that account for the first and second polarities of light, respectively. The grating spacing used in the first grating section of each grating subassembly may be within a range of about 0.1 to about 0.5 microns different from the grating spacing used in the second grating section of the same grating subassembly. Stated another way, the grating spacing used in the first and second grating sections of each grating subassembly differ by an amount that is of a range of 0.1 microns to about 0.5 microns. Adjacent grating subassemblies will have larger grating spacings to account for addressing different wavelengths of light. The grating spacings used by adjacent grating subassemblies will typically be at least about 1 micron different from each other.
A third aspect of the present invention generally relates to a method for making a demultiplexer. Multiple layers define a stack. xe2x80x9cOverlyingxe2x80x9d is used in relation to this third aspect to identify that a particular layer within the stack is disposed at a higher elevation than one or more other layers. Being characterized as xe2x80x9coverlyingxe2x80x9d should not be construed as meaning that there can be no intermediate layer. For instance, characterizing a first layer as being disposed in overlying relation to a second layer does not require that the first layer be disposed directly on top of the second layer, although it encompasses such a scenario. Moreover, the xe2x80x9coverlyingxe2x80x9d characterization does not require that a particular layer overly the entirety of another layer.
The method of the third aspect generally includes forming a first waveguide layer in overlying relation to a first substrate, forming a barrier layer in overlying relation to the first waveguide layer, and forming a second waveguide layer in overlying relation to the barrier layer. Generally, the demultiplexer defined by the method of the third aspect includes a first demultiplexing subassembly in the form of a grating assembly on one side of the demultiplexer (e.g., one and typically a plurality of grating subassemblies), and a second demultiplexing subassembly in the form of a photodetector assembly on an opposite side thereof (e.g., one and typically a plurality of spaced photodetectors). In this regard, either the first or second demultiplexing assembly is formed at least in part by processing a surface of the second waveguide layer that projects away from the underlying barrier layer. Thereafter, the assembly formed thus far is inverted and mounted on a second substrate such that the demultiplexing assembly formed on the second waveguide layer projects at least generally toward the second substrate. A cap, protective layer, or the like first may be disposed over this particular demultiplexing assembly and/or for providing for a more desirable interface with the second substrate. In any case, the first substrate (which is now xe2x80x9con topxe2x80x9d) is then removed to expose a surface of the first waveguide layer that is opposite that which interfaces with the barrier layer. The other of the first and second demultiplexing subassemblies may then be formed at least in part by processing this surface of the first waveguide layer.
Various refinements exist of the features noted in relation to the third aspect of the present invention. Further features may also be incorporated in the third aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. The formation of the first or second demultiplexing subassembly may entail forming an etch stop layer in overlying relation to the first substrate before forming the first waveguide layer. As such, the first waveguide layer would then be formed in overlying relation to both the etch stop layer and the first substrate. Removal of the first substrate could then be accomplished by mechanical polishing and chemical etching, with the etch stop layer being used to terminate the etching operation. The etch stop layer would thereafter be removed to allow for the processing to at least complete the definition of the first or second demultiplexing assembly directly on the first waveguide layer.
Each of the first waveguide layer, the barrier layer, and the second waveguide layer may be epitaxial. Representative ways in which the first waveguide layer, the barrier layer, and second waveguide layer may be formed include using molecular beam epitaxy, metal organic chemical vapor deposition, and liquid-phase epitaxy. One way in which the formation of the first waveguide layer, the barrier layer, and the second waveguide layer may be characterized is that no crystal regrowth need be utilized. Crystal regrowth may degrade the optical properties of the demultiplexer. Instead, the first waveguide layer, the barrier layer, and the second waveguide layer may each be entirely formed within the same processing chamber before ever having to remove this intermediate structure from the processing chamber for processing to complete the formation of the first and second demultiplexing subassemblies. Stated another way, no portion of either the first waveguide layer, the barrier layer, or the second waveguide layer is formed after the first and second demultiplexing subassemblies have been completely formed. Stated yet another way, all depositions that are used to define the first waveguide layer, the barrier layer, and the second waveguide layer are done without ever having to remove the substrate from a processing chamber for some intermediate processing. What is commonly characterized as xe2x80x9cback side etching techniquesxe2x80x9d may be utilized in relation to the methodology of the third aspect for allowing processing on effectively opposite sides of the demultiplexer to at least assist in the formation of the first and second demultiplexing subassemblies.
Formation of the grating assembly may be done by etching in the third aspect. Preferably, a controlled or timed etch is utilized to define the grating assembly. Stated another way, the etching which may be used to form the grating assembly may be executed without using any type of an etch stop layer. Typically, the grating assembly will be in the form of a plurality of grating subassemblies that are spaced in the general direction in which a combined or multiplexed optical signal will travel through the first waveguide layer in the finished demultiplexer. Conventional semiconductor processing techniques may be used to form the photodetector assembly. Typically, the photodetector assembly will be in the form of a plurality of photodetectors that arc also spaced in at least the general direction in which a combined or multiplexed optical signal will travel through the first waveguide layer in the finished demultiplexer. It should be appreciated that the method of the third aspect may be utilized to make the demultiplexers of the first aspect discussed above.
At least the initial portion of the formation of the photodetector assembly may be executed simultaneously with the formation of the first or second waveguide layer. That is, the same deposition process that may be employed to define the first or second waveguide layer may also be employed to define the photodetector assembly. In this regard, the photodetector assembly may define the entirety of the first or second waveguide layer. However, the formation of the photodetector assembly is not complete until after the deposition of the one or more layers which may define the first or second waveguide layer. That is, at least some processing is executed on an exposed surface of the first or second waveguide layer to complete the definition of the photodetector assembly. For instance, a patterning operation may be executed on this exposed surface to define a plurality of individual photodetectors that are spaced at least generally the direction in which the combined optical signal will travel through the demultiplexer that is made by the subject third aspect. Moreover, some processing on/through this surface will be required to establish electrical contact with the photodetector assembly.
A fourth aspect of the present invention relates to an optical switch that includes a pair of waveguides and at least one optically-induced grating of sorts for controlling which of these waveguides functions as the output channel for the optical signal that enters the optical switch. Generally, the optically-induced grating induces a spatially periodic change in the index of refraction in one of the waveguides to change the output channel from one of the waveguides to the other of the waveguides. Therefore, when the optically-induced grating is in a first mode, one of the waveguides is the output channel, while when the optically-induced grating is in a second mode, the other of the waveguides is the output channel.
Various refinements exist of the features noted in relation to the fourth aspect of the present invention. Further features may also be incorporated in the fourth aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. In one embodiment, the optical switch of the fourth aspect includes first and second waveguides that may be viewed as being in a stacked configuration, with the first waveguide being disposed a higher elevation in the stack than the second waveguide. xe2x80x9cStackedxe2x80x9d does not necessarily mean that the first waveguide is directly above the second waveguide or vice versa, although this typically will be the case.
What may be characterized as a barrier layer or index control channel may be and preferably is disposed between the first and second waveguides in the case of the fourth aspect. Selective coupling of the first and second waveguides effectively is the function of this barrier layer. Functionally, the barrier layer allows only certain light to pass from the first waveguide into the second waveguide when the optically-induced grating is in one of its two modes, and does not allow for any significant transmission of light through the barrier layer and into the second waveguide when the optically-induced grating is in its other mode.
Modification of the index of refraction in relation to the fourth aspect will be described in relation to an optical switch having first and second waveguides. An optical mask may be formed on an exposed external surface of the first waveguide, and at least one and more preferably a plurality of slits are formed in this optical mask. Multiple slits are preferably spaced in the direction in which light at least generally travels through the first waveguide. Preferably, the optical mask is opaque, although having limited transmissiveness may be tolerable in certain instances. Moreover, preferably each slit in the optical mask extends through the entire thickness of the optical mask to an exposed surface of the first waveguide, although retaining a certain amount of optical mask material at the bottom of each slit may be tolerable in certain instances. Stated another way, the optical mask has one degree of transparency, while the slits have a different, higher degree of transparency. In any case, a light source may be provided to project a control beam of sorts onto the optical mask in the area of the slit(s). The light source, the optical mask, and any slit formed in the optical mask may be characterized as being part of an optically-induced grating assembly.
Light from the light source that may be utilized by the fourth aspect may be directed toward the first waveguide to induce a general change the index of refraction of the first waveguide. This change in index of refraction provides the two modes for the optical switch associated with the fourth aspect, or more specifically for the optically-induced grating. In one embodiment: 1) when the light source is xe2x80x9coff,xe2x80x9d the first waveguide and the second waveguide are optically xe2x80x9cdecoupledxe2x80x9d and the optical signal will progress only through the first waveguide such that it is the output channel (i.e., the optical signal will not pass through any separation or barrier layer and into the second waveguide in this instance); and 2) when the light source is xe2x80x9conxe2x80x9d such that light is directed onto the noted optical mask and into at least one of its slits and onto the first waveguide, the index of refraction of the first waveguide will be modified in such a manner such that the optical signal from the first waveguide will be able to pass through the barrier layer and into the second waveguide such that the second waveguide is the output channel in this instance. It may also be possible to xe2x80x9creversexe2x80x9d the foregoing in relation to which of the first and second waveguides is the output channel when the light source is on or off.